Photolithographically patterned, stress-engineered spring structures have been developed, for example, to produce low cost probe cards and to provide electrical connections between integrated circuits. A typical conventional spring structure is formed from a stress-engineered (a.k.a. “stressy”) film intentionally fabricated such that its lower/upper portions have a higher internal tensile stress than its upper/lower portions. This internal stress gradient is produced in the stress-engineered film by layering different materials having the desired stress characteristics, or using a single material by altering the fabrication parameters. The stress-engineered film is patterned to form fingers that are secured to an underlying substrate either directly or using an intermediate release material layer. When the release material (and/or underlying substrate) is selectively etched from beneath a cantilever (free) portion of the spring finger, the cantilever portion bends away from the substrate due to a bending force generated by the internal stress gradient, thereby producing a curved spring finger that remains secured to the substrate by an anchor portion. Such spring structures may be used in probe cards, for electrically bonding integrated circuits, circuit boards, and electrode arrays, and for producing other devices such as inductors, variable capacitors, and actuated mirrors. Examples of such spring structures are disclosed in U.S. Pat. No. 3,842,189 (Southgate) and U.S. Pat. No. 5,613,861 (Smith), both being incorporated herein by reference.
Most spring structures of the type mentioned above are currently produced using sputter deposition techniques in which a stress-engineered thin film is sputter deposited while changing the sputtered composition or the process parameters (e.g., pressure and/or power) during the deposition process, and this sputter-deposition production method has proven reliable for generating suitable stress-engineered films in small batches. However, the mass production of low-cost spring structures by the sputter-deposition production method faces several obstacles. First, the equipment needed to perform sputter deposition is very expensive to purchase and maintain. Second, sputter-deposition production methods have proven complex in part due to the tendency for process parameters of the sputtering equipment to drift (change) over time during repeated sputter runs, which requires constant recalibration of the sputtering equipment. Further, localized differences in the process parameters within the sputter equipment can result in significant differences in the tip heights of spring structures formed during a particular sputter run, thereby yielding inconsistent and unpredictable results. Moreover, suitable sputter equipment that allows for high stress uniformity over large (i.e., greater than four inch diameter) wafers has not yet been produced.
One recently developed alternative to sputter-based fabrication techniques is the use of plating techniques during which process parameters are changed to produce the desired stress gradient. Such plating-based fabrication methods are attractive not only because of the much lower equipment cost, but also because plating can be reliably scaled to large substrate sizes. However, although plating methods facilitate the fabrication of plated films exhibiting internal stress gradients, the attainable stress values in the compressive region of the stress-engineered film are relatively low, and plated stress-engineered films often exhibit weak adhesion to the underlying substrate.
What is needed is a highly reliable and repeatable method for fabricating spring structures that exhibit the relatively high tip heights of sputtered, stress-engineered spring structures, but avoids the associated high manufacturing expense and complexity.